1. Field of the Invention
The present invention relates to an improvement in a solid state color imaging apparatus.
2. Description of the Prior Art
A solid state image sensor is, as is well known, an apparatus for converting objective image to an electric signal. A single chip solid state color camera is an apparatus comprising color filters of different colors in front of the solid state color image sensor, thereby to obtain an objective image on the surface of the solid state image sensor, which produces spatially modulated chrominance signal and luminance signal, enabling to synthesize a composite color signal.
On the other hand, the solid state image sensor is a kind of a large scale integrated circuit (LSI), and therefore in order to obtain a high production yield, degree of the integration of the device should be preferably low. Therefore, the number of the photoelectric elements, disposed in a two dimensional pattern on a substrate, that is the number of picture element, should be as small as possible under a condition for attaining a required resolution power of image. Accordingly, the way of producing chrominance signal should be that which does not lower theoretical value of the resolution attainable for the number of picture elements. Many proposals have been made for the modes of the pattern combination between the picture elements and the color filter thereof, and main stream thereof are those using mosaic color filter.
On the other hand, the solid state image sensor are on a stream of higher integration in order to meet a requirement of higher productivity and manufacturing yield, and according thereto the color filter to be combined therewith should also be of higher integrated pattern. This raises a hard problem that manufacture of the color filter in more highly integrated mosaic pattern is very difficult; and a more hard problem is the registration between the highly integrated picture elements on the solid state image sensor and the highly integrated color filter, a defect of which hitherto has resulted in poor quality of picture signal.
In this respect, an improvement of the solid state color imaging apparatus, wherein the registration of the color filter to the solid state image sensor is easier than conventional system, has been waited for.
The problems in the conventional type solid state color imaging apparatus are elucidated referring to FIGS. 1 to 3. FIG. 1 shows a fundamental type apparatus which uses a color filter having filter pattern of vertical color stripes comprising repetition of three primary colors in a horizontal direction. The apparatus has a solid state image sensor having a number of photoelectric elements, i.e., picture elements 101 disposed forming vertical rows and horizontal rows, and a color filter having red filter stripes 102, green filter stripes 103 and blue filter stripes 104 disposed in turn. In the signal produced based on sampling by the picture elements of the objective image which is spatially modulated by the color filter stripes, the chrominance signals for colors of the color filter are modulated by respective color filter stripes. The carrier frequency of the chrominance signal is one third (1/3) of the sampling frequency by the picture elements. In such apparatus, for objective image of a low color purity, all the picture elements on one horizontal line are sensitive, and therefore the sampling of the objective image is made by using all the picture elements on the horizontal line. In this case, the band-width of signal of the objective image sampled by picture elements is a half (1/2) of the sampling frequency of the picture elements. Therefore, the carrier of the chrominance signal is included within the band width of the luminance signal. In order to avert the undesirable inclusion of the chrominance signal carrier in the luminance signal band, the luminance signal band width has to be limited under 1/3 of the sampling freuency, resulting in a disadvantage of lowering of horizontal resolution.
In order to remove the above-mentioned shorting of the first conventional type apparatus elucidated in FIG. 1, the assignee corporation (intended patentee) of the present invention has proposed another type apparatus shown by FIG. 2 (Japanese Patent Application No. Sho 51-126592, published in the Japanese Patent unexamined publication No. Sho 53-50923. In the apparatus of FIG. 2, nH designates an n-th horizontal scanning line of a first field of a frame, nH' designates an n-th horizontal scanning line of a second field of the frame, (n+1)H and (n+1)H' designate those of (n+1)-th, respectively, and so on. In this apparatus, by the n-th horizontal scannings a first color differential signal is produced, and by the (n+1)-th horizontal scannings a second color differential signal is produced, and so on. Besides, in order to make average values of liminance signal of the horizontal scannings uniform, each picture elements are divided into two parts which are covered by different color filters. For example, a picture element 201 of FIG. 2 is covered by red light passing filter element (hereinafter referred as red filter element) 202 and red stopping filter element (hereinafter referred as cyan filter element) 203 and a picture element next thereto is covered by green filter element 202' and cyan filter 203. In this example of FIG. 2, frequency of repetition of the filter elements in the horizontal direction is a half (1/2) of the frequency of repetition of the picture elements in the horizontal direction. Accordingly, the carrier frequency of spatially modulated chrominance signal becomes half of sampling frequency sampling the image object, which sampling frequency is determined by number of the picture elements.
As elucidated above, the imaging apparatus of FIG. 2 has advantage of higher resolution than the apparatus of FIG. 1. However, the problem of the construction of FIG. 2 is requiring a mosaic pattern (not of simple stripe pattern) filter and a high accuracy registration of the filter mosaic pattern and the mosaic picture element pattern. The latter requirement of registration the apparatus of FIG. 2 is elucidated in detail further referring to the figures of FIGS. 3(A), 3(B) and 3(C) exemplifying a small part of the apparatus. FIG. 3(A) shows a normal or ideal state where the filter elements and the picture elements are of accurate registration, FIG. 3(B) shows a state where the registration slips off in the horizontal direction and FIG. 3(C) shows a state where the registration slips off in the vertical direction. In these figures, the numerals 301, 302, 303 and 304 exemplify picture elements, wherein the picture elements 301 and 302 are those to be read out in an n-th horizontal scanning of a first field and the picture elements 303 and 304 are those to be read out in an n-th horizontal scanning of a second field of the same frame.
In the normal registration state of FIG. 3(A), the output signals S.sub.301, S.sub.302, S.sub.303 and S.sub.304 of the picture elements 301, 302, 303 and 304, respectively are given as follows: ##EQU1## where
R is electric signal based on red light irradiated through the red filter element.
G is electric signal based on green light irradiated through the green filter element.
.DELTA.Cy is electric signal based on cyan light irradiated through the cyan filter element. Mark .DELTA. is attached to indicate the electric signal based on cyan filter which coveres only a small fraction of the picture element.
Accordingly, luminance signals Y.sub.nH and Y.sub.nH, and the color difference signals D.sub.nH and D.sub.nH' of respective fields become as follows: ##EQU2##
Thus the luminance signals of respective fields as well as color difference signals of respective fields are equivalent each other in the normal state of registration.
Next, the case of lateral slip off of the registration shown in FIG. 3(B) is elucidated. In this case, signal components dR and dG inherent to small part of different color filters on the picture elements appear as shown in FIG. 3(B), and therefore, the output signals S.sub.301h, S.sub.302h, S.sub.303h and S.sub.304h of the picture elements 301, 302, 303 and 304, respectively, becomes as follows: ##EQU3##
Accordingly, the luminance signals Y.sub.nHh and Y.sub.nH'h and the color difference signals D.sub.nHh and D.sub.nH'h of respective fields become as follows: ##EQU4## As shown in the above equations, the color difference signals D.sub.nHh and D.sub.nH'h have second terms -(G-2dG), in comparison with the counterparts D.sub.nH and D.sub.nH' of the equation (2), and this means that the colors differential signals are lowered as a result of the horizontal slip off of the registration.
Next, the case of vertical slip off of the registration shown in FIG. 3(C) is elucidated. In this case, signal components .delta.R, .delta.G and .delta.Cy inherent to small part of different color filter on the picture elements appear as shown in FIG. 3(C), and therefore, the output signals S.sub.301v, S.sub.302v, S.sub.303v and S.sub.304v of the picture elements 301, 302, 303 and 304, respectively, becomes as follows: ##EQU5##
Accordingly, the luminance signals Y.sub.nHv and Y.sub.nH'v and the color difference signals D.sub.nHv and D.sub.nH'v of respective fields becomes as follows: ##EQU6## As shown in the above equations, the color difference signals D.sub.nHv and D.sub.nH'v have different send terms (G+.delta.G) and (G-.delta.G) from each other. And furthermore, the luminance signals Y.sub.nHv and Y.sub.nH'v of the equations (6) are different from each other. Such differences between the signals of the first field and the second field leads to considerable flickering of luminance and chrominance. The same applies to the (n+1)-th scanning.
The above-mentioned lowering of the color differential signal and occurrence of flickering are shortcomings which is likely to occur in the apparatus using a mosaic type composite filter.